I placed a bid on this item for
£52

https://www.ebay.co.uk/itm/163501004892

but the winning bid was
£42
and my bid isn't showing.

Am I doing something wrong

You've done everything properly.

Someone purchase the item using Best Offer and that ended the listing earlier.
You can't bid on ended listings and so the sniper. eBay will not show bids in this case as it did not accept it.